The present invention relates in general to object location and tracking systems that identify identification of locations of radio-tagged objects, and is particularly directed to the use of a periodically exercised, reference tag-based mechanism for removing timing errors associated with cable plant and other components of signal transport paths between a plurality of geographically distributed readers and an object location processor. The object location processor executes time-of-arrival differentiation of first-to-arrive transmissions from tags as detected by the readers. Removal of timing errors ensures that the object location processor will precisely geolocate a tagged object in the field of view of the readers.
As described in the introductory portion of the above-referenced ""710 application, the U.S. Patent to Heller, U.S. Pat. No. 5,119,104, entitled: xe2x80x9cLocation System Adapted for Use in Multipath Environmentsxe2x80x9d describes a motion-based system for tracking objects xe2x80x98taggedxe2x80x99 with micro-miniaturized radio transmitters, that are normally in a quiescent mode, until triggered by associated motion sensors. When the object is moved, a motion sensor causes its tag transmitter to emit an RF signal encoded with the identification of the tag; as long as the object is moving, its tag will transmit. Using multi-lateration receivers distributed in the monitored area of interest, and referenced to a time base for time-of-arrival processing, the location of a radio tag and thereby its object can be tracked, while the object is being moved, up to the point where it is at rest. The tag radio then reverts to quiescent mode, with transmission disabled until the object is again moved.
A principal shortcoming of such a motion-dependent object tracking system is the fact that, in addition to being dependent up the object being moved, and contrary to what the patent alleges, the patented system does not effectively solve the problem of multipath inputs to its tracking receiver subsystem. This latter shortcoming is due to the fact that it employs relatively simple amplitude detection receivers that operate on the assumption that the strongest signal will be the first-to-arrive signal. This means that the Heller approach will erroneously use a later arriving, large amplitude, multipath signal, rather than a relatively weak, but first-to-arrive signal, that has traveled to the receiver in a direct path through an attenuating medium.
A further deficiency of the system proposed in the Heller patent is the fact that it is not concerned with the more fundamental problem of asset management. Asset management not only addresses the need to locate and track processed components in the course of their travel through a manufacturing and assembly sequence, but is also concerned with the more general problem of component and equipment inventory control, where continuous knowledge of the whereabouts of any and all assets of a business, factory, educational, military or recreational facility, and the like, is desired and/or required. An asset management system may also benefit from status information that can be provided to the tag, by means of an auxiliary sensor associated with the tagxe2x80x94something not addressed by the Heller scheme.
Advantageously, the deficiencies of conventional object location systems, such as that proposed in the Heller patent, are successfully remedied by tagged object geolocation systems of the type described in the U.S. Patents to Belcher et al, U.S. Pat. Nos. 5,920,287, and 5,995,046, assigned to the assignee of the present application and the disclosures of which are incorporated herein.
The overall architecture of these significantly improved tagged object geolocation systems is shown diagrammatically in FIG. 1 as comprising a plurality of tag emission readers 10 geographically distributed within and/or around an asset management environment 12. The environment contains a plurality of objects/assets 14, whose locations are to be monitored on a continuous basis and reported to an asset management data base 20, which is accessible by way of a computer workstation or personal computer, as shown at 26. Each of the tag emission readers 10 monitors the asset management environment for emissions from one or more tags 16 affixed to the objects 14. Each tag 16 repeatedly transmits or xe2x80x98blinksxe2x80x99 a very short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of its associated object and other information stored in tag memory.
For this purpose, the tag emission readers 10 are installed at fixed (precisely geographically known), relatively unobtrusive locations within and/or around the perimeter of the environment, such as doorway jams, ceiling support structures, and the like. Each tag reader 10 is coupled to an associated reader output processor of an RF processing system 24, which is operative to correlate the spread spectrum signals received from a tag with a set of spread spectrum reference signal patterns, and thereby determine which spread spectrum signals received by the reader is a first-to-arrive spread spectrum signal burst transmitted from the tag.
The first-to-arrive signals extracted by reader output processors from the signals supplied from the tag emission readers 10 are coupled to an object location processor within the processing system 24. The object location processor performs time-of-arrival differentiation of the detected first-to-arrive transmissions, and thereby locates (within a prescribed spatial resolution (e.g., on the order of ten feet) the tagged object of interest.
In order to mitigate against the potential for fades and nulls resulting from multipath signals destructively combining at one or more readers, the geolocation system of FIG. 1 may be augmented to employ a spatial diversity-based receiver-processing path architecture. In accordance, with this architecture, rather than employ a single RF signal processing path for each reader location, a plurality of readers (e.g., two readers) are installed at each monitoring location, and associated signal processing paths are coupled therefrom to the geometry (triangulation) processor.
FIG. 2 diagrammatically shows a non-limiting example of this augmented geolocation system in which a plurality (e.g., two) of tag emission readers are located at geographically distributed monitoring locations, three of which are shown at 101, 102, 103. Monitoring location 101 has first and second tag readers 101-1 and 101-2, whose respective output signal processing paths include first arrival detector units 111-1 and 111-2. Coupled with the RF signal processing circuits of the front ends of the tag readers 101-1 and 101-2 are antennas 2101-1 and 2101-2. To provide spatial diversity-based mitigation of multipath signals, the antennas 2101-1 and 2102-1 are spaced apart by a distance sufficient to effectively statistically minimize destructive multipath interference at both antennas simultaneously.
For the other two monitoring locations of FIG. 2, monitoring location 102 has first and second spatially diverse antennas 2102-1 and 2102-2, which feed tag readers 102-1 and 102-2, whose outputs are coupled by way of first arrival detector units 112-1 and 112-2 to triangulation geometry processor 400. Similarly, monitoring location 103 has first and second spatially diverse antennas 2103-1 and 2103-2, which feed tag readers 103-1 and 103-2, coupled to tag readers 103-1 and 103-2, the outputs of which are coupled by way of first arrival detector units 113-1 and 113-2 to the triangulation geometry processor 400.
The triangulation geometry processor 400 employs a standard multi-lateration algorithm that relies upon time-of-arrival inputs from at least three detectors (in the example of FIG. 2, three detector unit pairs 111-1/111-2; 112-1/112-2; and 113-1/113-2) to compute the location of the tagged object 16. The multi-lateration algorithm executed by processor 400 employs a front end subroutine that selects the earlier-to-arrive outputs of the detector pairs 111-1/111-2; 112-1/112-2; and 113-1/113-2, as the value to be employed in the multi-lateration algorithm. Because of the use of spatial diversity, there is an extremely high probability (on the order of ninety percent or greater) that at least one of the two readers 10i-1 and 10i-2 at any given reader location 10i will provide a first-to-arrive output value to the processor 400 for any tag emission.
FIG. 3 diagrammatically shows a modification of the embodiment of FIG. 2, in which a plurality of auxiliary xe2x80x98phased arrayxe2x80x99 signal processing paths (four of which are shown at 13i-1, 13i-2, 13i-3 and 13i-4) are coupled to the antenna pair 210i-1 and 210i-2, in addition to the paths containing the readers 10i-1, 10i-2, and their associated first arrival detector units 11i-1 and 11i-2 that feed the processor 400. Each phased array path 13i-j sums energy received from the two antennas in a prescribed phase relationship, with the energy composite being coupled to the associated readers and detector units that feed the processor 400.
The phased array architecture of FIG. 3 addresses the situation in a multipath environment where a relatively xe2x80x98earlyxe2x80x99 signal may be canceled by an equal and opposite signal arriving from a different direction. Advantage is taken of the array factor of a plurality of antennas to provide a reasonable probability of effectively ignoring the destructively interfering energy. The phased array provides each reader site with the ability to differentiate between received signals, by using the xe2x80x98patternxe2x80x99 or spatial distribution of gain to receive one incoming signal and ignore the other.
Regardless of the geolocation architecture employed, a typical installation will contain varying lengths of cable plant (such as RF coax) that connect the readers to the RF processor. In some cases, the cables can be very short and may be indoors. In other cases, at the same site, the cables can be very long and may be outdoors. This differential cable length and environment parameter situation creates the possibility of system timing errors, associated with the cable delays drifting due to weather or other effects (e.g., age, humidity, temperature, physical stretching, etc.), resulting in geolocation errors.
In accordance with the present invention, this signal transport delay problem is effectively obviated by placing one or more xe2x80x98referencexe2x80x99 tags, whose geolocations, like those of the tag emission readers, are fixed within the monitored environment containing the objects to be tracked, and precisely known. Using a background calibration routine that is exercised at a relatively low cycle rate, emissions from the reference tags are processed and coupled to the geolocation processor. The calculated geolocations of the reference tags are compared with their actual locations, which may be stored in a calibration database or stored in memory on board the reference tag and included as part of the information transmitted by the reference tag and received by transmission readers. Any offset between the two geolocation values (measured and actual) is used to adjust the time delays of the various (cable plant) signal transport paths between the readers and the geolocation processor, and thereby track out associated timing errors.